Thermochemical Reaction

Question

Consider the following generic thermochemical reaction: 2A + 3B → 2C  ∆Hrxn = -100 kJ

If a reaction contains 5 mol A and 6 mol B, how much heat will have evolved once the reaction has gone as far as it can go?

a. 150 kJ

b. 200 kJ

c. 250 kJ

d. None of these.

Answer 1

2A + 3B ==> 2C ∆Hrxn = -100 kJ

Note that the -100 kJ refers to the reaction as written, i.e. it is for 2 moles of A and 3 moles B, etc.

You are given 5 moles A and 6 moles of B, so let's see which one is limiting.

If you divide the moles of reactant by the coefficient in the balanced equation, you can find which reactant is limiting. In our case we have the following:

5 moles A ÷ 2 = 2.5

6 moles B ÷ 3 = 2

Therefore B will be limiting because 2 is less than 2.5

Now we can answer the question.

6 moles B x -100 kJ/3 moles B = -200 kJ for the reaction. So, the answer would be (d) none of these, because none of them have a negative value.